The differentiated cell state is often considered stable and resistant to changes in lineage identity. However, differentiated somatic cell types from humans and other organisms have been reprogrammed to the pluripotent state (“pluripotent reprogramming”) by forced expression of a set of transcription factors (Takahashi, K. et al. Induction of pluripotent stem cells from adult human fibroblasts by defined factors, Cell 131, 861-872 (2007)), somatic cell nuclear transfer (Campbell et al., Sheep cloned by nuclear transfer from a cultured cell line, Nature 380: 64-66 (1996); Gurdon et al., Sexually mature individuals of Xenopus laevis from the transplantation of single somatic nuclei, Nature 182, 64-65 (1958)) or cell fusion (Cowan et al., Nuclear reprogramming of somatic cells after fusion with human embryonic stem cells, Science (New York, N.7309, 1369-1373 (2005); Tada et al., Nuclear reprogramming of somatic cells by in vitro hybridization with ES cells, Curr Biol 11, 1553-1558 (2001)). Additionally, a few studies have demonstrated that through ectopic expression of selected genes or by cell fusion, an adult cell type can be directly converted to another adult cell type (Cobaleda et al., Conversion of mature B cells into T cells by dedifferentiation to uncommitted progenitors, Nature 449, 473-477 (2007); Davis et al., Expression of a single transfected cDNA converts fibroblasts to myoblasts, Cell 51, 987-1000 (1987); Feng, et al. PU. 1 and C/EBPalpha/beta convert fibroblasts into macrophage-like cells, Proc. Nat. Acad. Sci. USA 105, 6057-6062 (2008); Ieda et al. Direct reprogramming of fibroblasts into functional cardiomyocytes by defined factors, Cell 142, 375-386 (2010); Zhou et al., In vivo reprogramming of adult pancreatic exocrine cells to beta-cells, Nature 455, 627-632 (2008); and Zhou, Q. & Melton, D. A. Extreme makeover: converting one cell into another, Cell Stem Cell 3: 382-388 (2008)). This process is termed trans-differentiation or lineage reprogramming.
However, major challenges remain due to the low efficiency and slow reprogramming process. A more significant challenge is how to accomplish cell reprogramming with minimal genetic changes in the reprogrammed cells, because such genetic changes give rise to concerns about introduced mutations at the insertion site of expression cassettes encoding pluripotency factors and other genetic material.